The present invention is related to photolithography. More specifically, the invention relates to a system and method for correcting rule violations of a digital representation of a photomask, especially after the digital representation has been corrected for optical proximity.
In the fabrication of integrated circuits, the process of converting a desired electrical circuit schematic into patterns to be imaged on an integrated circuit (“IC” or “chip”) involves many steps. The electrical circuit schematic is first converted into a desired chip layout having a set of patterns to be produced on a semiconductor chip. Such chip layout typically includes conductor patterns, active semiconductor area patterns and isolation area patterns at a lowest and most densely patterned level of the chip. The chip layout also includes wiring patterns at higher levels of the chip. These patterns are typically produced on the chip by photolithographic imaging onto a photoresist layer through several photomasks (hereinafter, “masks”), each mask having a set of mask patterns for producing the patterns in the photoresist.
A photolithographic imaging system includes an illumination source, a mask, and lenses for focusing an image produced by the mask onto a photoresist layer on a substrate such as a semiconductor wafer.
The circuit layout is densest and the patterns are the smallest at the lowest (semiconductor) level of the wafer. The patterns are so small that the images produced by the photolithographic imaging system are near the limit of resolution of the imaging system. Diffraction can cause the light passing through the openings of the mask to interfere constructively and destructively.
Because of this, the smallest patterns on a wafer generally cannot be printed directly by images of the same patterns on a mask. The imaging system simply cannot produce exactly the same image on the wafer that appears on the mask. A phenomenon known as “photolithographic line end shortening” can occur, by which the ends of line patterns appear shorter as imaged in the photoresist than they do on the mask. The image can also vary based on whether patterns are spaced at even spacings from each other or at irregular spacings.
Thus, the degree of change in the image from the mask patterns to the photoresist patterns is a function of the proximity of patterns on the mask in view of the size of the feature to be printed and the resolution limit of the imaging system.
Optical proximity correction (OPC) is a method used to correct for the above effect to generate the patterns of a mask. The goal of OPC is to generate a set of mask patterns that will result in the desired image on a wafer. Typically, a data set representing a layout pattern is provided to a computer system executing an OPC program. The OPC program predicts changes that would occur in the photoresist image if the mask contained the same layout pattern. The layout pattern of the mask is then altered by the OPC program in a way that is predicted to produce the correct image on the wafer. All of these steps are performed according to a program executed on a computer with respect to a data set representing the layout pattern.
For example, as shown in FIG. 1, a layout pattern includes a plurality of design shapes including the shapes 10, 12, 14 and 16 and 18. Because of optical proximity, the design shapes may not be capable of being printed by a set of patterns of a mask having the same shapes. Accordingly, the layout pattern is processed according to an OPC program to generate a set of mask shapes that will result in a desired layout pattern on the semiconductor wafer. FIG. 2 illustrates the corresponding mask shapes 200, 202, 204, 206 and 208 that result after OPC processing. The contours of the original design shapes 10, 12, 14, 16 and 18 are indicated in FIG. 2 by dotted line.
However, the result after OPC processing may still not be usable. This is because OPC processing is only concerned with the layout pattern to be produced on the wafer, but not the patterns of the mask. Each mask type has a set of rules which define the limits of how patterns can be formed thereon. Pattern elements must conform to the mask rules. If the pattern elements do not conform to the rules, the mask cannot be made. After OPC processing, the data set representation of the mask patterns may contain elements that cannot be fabricated on the mask. Such elements are referred to as “mask rule violations”.
Therefore, a system and method is needed by which mask data representing OPC corrected mask shapes is corrected for mask rule violations.
It would be desirable to provide a system and method of generating mask pattern data following OPC that quickly arrives at final mask patterns.
It would further be desirable to provide a method for correcting mask rule violations in OPC corrected mask data that is expected to finish within a finite number of iterations.
It would further be desirable to provide a method for correcting mask rule violations in OPC corrected mask data that gradually reverses OPC corrections to remove the mask rule violations.